The present invention relates to ferromagnetic thin-film memories and, more particularly, to ferromagnetic thin-film memories in which information states of the memory cells are based on alternative magnetization directions along the easy axis of the thin-film.
Digital memories of various kinds are used extensively in computers and computer system components, digital processing systems, and the like. Such memories can be formed, to considerable advantage, based on the storage of digital system bits as alternative states of magnetization in magnetic materials in each memory cell, typically ferromagnetic thin-film materials. The information stored in such films is stored therein through selected directions of magnetization occurring in that film, this information being obtainable from such films through either inductive sensing to determine the magnetization state, or by magnetoresistive sensing of such states. Ferromagnetic thin-film memories of this nature may be conveniently provided on the surface of a monolithic integrated circuit chip to provide easy electrical interconnections between the memory cells and the memory operating circuitry provided in the chip.
Ferromagnetic thin-film memory cells can be made very small and they can be packed very closely together to achieve a significant density of stored information bits, properties which are the basis of permitting them to be provided on the surface of a monolithic integrated circuit, as indicated above. Suitable constructions for such cells can be found in U.S. Pat. Nos. 4,731,757 to Daughton et al entitled "Magnetoresistive Memory Including Thin Film Storage Cells having Tapered Ends" and U.S. Pat. No. 4,780,848 to Daughton et al entitled "Magnetic Memory" and an earlier filed application by J. M. Daughton having U.S. Pat. No. 4,918,655 entitled "Magnetic Device Integrated Circuit Interconnection System," all of which have been assigned to the same assignee as the present application and are herein incorporated by reference. A structure analogous in some respects to those disclosed there is shown, as an example, in FIG. 1.
There, a bit structure, 10, from a series string of them in a monolithic integrated circuit is presented in a portion of that integrated circuit such that the opposite ends thereof, 11, are exposed. Bit structure 10 is shown formed over a semiconductor material body, 12, in and on which the monolithic integrated circuit is formed. Bit structure 10 is provided directly on an insulating layer, 13, supported on a major surface of body 12 in the integrated circuit chip. As indicated above, just a portion of the integrated circuit is shown, and then only a small part of the semiconductor body for that portion of the integrated circuit is shown in the figure.
Semiconductor material body 12 is typically formed of a doped silicon material of primarily either p- or n-type conductivity, with other regions of opposite conductivity provided therein to form circuit elements or portions of circuit elements. Insulating layer 13 is typically formed of silicon nitride both as an electrical insulator and as an oxygen barrier to prevent any oxygen from migrating therefrom into bit structure 10 and oxidizing any of the magnetic materials therein.
Bit structures like bit structure 10 are usually provided, as previously indicated, in a memory as a series string of such bit structures extending both ways from ends 11 where exposed at the edges of the integrated circuit portion shown in FIG. 1. The bit structures in series are connected to adjacent ones thereof in such an arrangement at junctures, 14, provided at each end of each of such bit structures where they provide an electrical short circuit for interconnecting them, and at the ends of each series string for interconnecting it to other circuit components in the monolithic integrated circuit operating the memory.
The remainder of bit structure 10 disposed on the exposed major surface of insulating layer 13 is comprised of a lower ferromagnetic thin-film, 15, and an upper ferromagnetic thin-film, 16. Ferromagnetic thin-film layers 15 and 16 exhibit uniaxial anisotropy, magnetoresistance, little magnetostriction, and are of an alloy composition typically comprising nickel, cobalt and iron. Typically, proportions of each are chosen to strongly reduce or eliminate any magnetostrictive effects in the films, and to improve certain other properties of them for the application.
Between ferromagnetic thin-film layers 15 and 16 is a further thin layer, 17, which usually would not be one exhibiting ferromagnetism, but may be either an electrical conductor or an electrical insulator. Layer 17 must, however, in this construction, prevent the exchange interaction between electron spins on neighboring atoms from coupling across the separation between layers 15 and 16 to thereby lock together the magnetizations of each. A typical choice for layer 17 would be tantalum doped with nitrogen.
A further diffusion barrier and protective layer, 18, is provided over upper ferromagnetic thin-film 16. This layer again can be formed of nitrogen doped tantalum. In junctures 14 there is further provided two other conductive layers, the first layer, 19, being formed of copper doped aluminum on which is provided a second layer, 20, of tungsten. These layers provide an electrical short between adjacent bit structures as indicated above.
An insulating layer, 21, is then provided to electrically isolate bit structure 10 from further structures to be provided thereabove. Layer 21 is typically formed of silicon nitride. A word line conductor, 22, is then provided over layer 21 extending to the tapered portions at the ends of bit structure 10. Typically, word line 22 extends over several similar bit structures, each one being in a different series string of such bit structures. Both of layers 21 and 22 are only partially shown in FIG. 1, with other portions thereof having been removed from the right-hand portion of bit structure 10 in that figure so that portion of bit structure 10 can be more clearly seen. Word line conductor 22 is typically formed with a thin layer of titanium and tungsten followed by a layer of copper doped aluminum. A passivation layer, 23, of silicon nitride is then provided over the entire structure, though this layer is also only partly shown, to protect it from external contaminants.
Bit structure 10 can be operated in a longitudinal mode having its easy axis extend between junctures 14 perpendicular to the direction of extension of word line conductor 22, or in a transverse mode having its easy axis of magnetization parallel with the direction of extension of word line conductor 22. In either situation, information, kept as a binary bit having one of two alternative logic values in bit structure 10, is stored therein in layers 15 and 16 by having the magnetization oriented to point in one direction or the other (but opposite in each of these layers to the direction in the other), generally along the easy axis of magnetization. If the direction of magnetization is caused to rotate from such a direction by external magnetic fields, the electrical resistance of layers 15 and 16 change with this magnetization direction rotation because of the magnetoresistive properties of such layers.
Near the edges of films 15 and 16, assuming that the easy axis is parallel with word line 22, anisotropy fields are dominated by the demagnetization fields due to the "free poles" at these edges. If the magnetizations of films 15 and 16 were saturated, the demagnetization fields would approach half the saturation fields, or perhaps 5,000 Oersteds, for these films with the alloys described here. Typical films of these alloys will have a coercivity and an anisotropy field on the order of only 20 Oersteds, leading to instabilities in the magnetization at the edges of these films.
In such large demagnetization fields, electron spins at the edge of the films are constrained to lie nearly parallel to these edges and the direction of elongation of these films. The directions of these electron spins only gradually turn to pointing across the films further inward toward the center of the films with the demagnetizing fields no longer overcoming the isotropy field. The rate, shape and distance of the occurrence are all a complex function depending on the magnetostatics of the situation, the quantum exchange interaction between adjacent atom electron spins, and the anisotropy considerations not unlike those leading to Neel walls.
Thus, the magnetization along the transverse axis is at or very nearly at zero at the outer edges of the films where word line 22 crosses them, and gradually increases in inward locations which are closer to the interior of the films, the magnetization value increasing toward the saturation value occurring in the central portions of the films. In the regions between the portions adjacent to the exterior edges of the film and the point where magnetic saturation begins, the film magnetizations are in transition from pointing along the direction of elongation to pointing along the direction of the easy axis. Therefore, such ferromagnetic thin-films having their easy axes extending in directions transverse to the direction of elongation thereof, or parallel to word line 22, do not truly saturate across the films along the easy axis but only across a part of such a film.
A representation of the magnetizations of a section of one of films 15 and 16 from between its tapered parts is shown in FIGS. 2A and 2B. FIG. 2A shows the magnetizations for the storage of a "0" logic value bit of information with the magnetizations at central locations shown pointed upward in that figure and edge magnetizations therein shown pointed primarily to the right. FIG. 2B shows the magnetizations for the storage of a "1" logic value bit of information with the magnetizations at central locations shown pointed downward in that figure, and with edge magnetizations therein shown still pointed primarily to the right.
Sense current refers to the current flow through bit structure 10 from one juncture 14 to the other juncture 14 thereof, and word current refers to current flowing in word line 22 adjacent to, and extending in a direction traverse to the direction of elongation of, bit structure 10. Bit structure 10 can be placed in one of two possible magnetization states along an easy axis thereof as shown in FIGS. 2A and 2B through the selective application of sense and word currents, i.e. information can be stored or "written" into bit structure 10.
Bit structure 10, in the configuration shown in FIG. 1 or in other typical configurations, in the past was typically placed in a "0" logic value magnetization state on the basis of providing a word current of typically 10.0 mA to 30.0 mA flowing in a direction which creates a magnetic field in films 15 and 16 oriented in the same common direction as the edge magnetizations of those films primarily were. A current through word line 22 giving such a result in a magnetic field will be termed a positive word line current or positive word current. Previous to this current flow, the edge magnetizations of films 15 and 16 were set by the application of a strong external magnetic field of typically a few thousand Gauss. The setting of a "0" logic value magnetization state was then completed by coincidentally providing a sense current of typically 2.0 mA to 3.5 mA coincidentally with the provision of the word current.
The opposite magnetization state representing a "1" logic value magnetization state was alternatively set into bit structure 10 through providing the same word current in the same direction, i.e. a positive word current, but providing a sense current of the same magnitude in the opposite direction through bit structure 10. Such magnetization state changes will occur very quickly after the proper current levels are reached, such changes between such states occurring in less than about 10 ns.
This method of establishing a magnetization state in memory films 15 and 16 of the bit structure 10 is generally a reliable way of doing so. However, there is on occasion found to be erroneous information in a bit structure 10 after such a storage procedure. Typically, the provision of a word current and a sense current is done to change the magnetization state of the bit structure 10 to the opposite state. The electron spins are reoriented under word line 22, at least in the central parts of films 15 and 16. As this occurs, the magnetic walls that initially form between the portions of films underneath word line 22 and the end portions thereof outside of word line 22 quickly propagate through the tapered portions to the ends of the films where the taper ceases. This occurs because the energy in the magnetic wall is dependent on the length thereof, and so having the wall propagate into the tapered end portions of bit structure 10 necessarily shortens the walls to thereby reduce the energy thereof. Since magnetic systems tend to minimize the magnetic energy thereof in accord with thermodynamics, the magnetic walls have a strong tendency to propagate to the ends of the tapered sections.
However, there is evidence that in some instances an initial magnetic wall, formed at the initiation of a storage procedure between the memory film portions under word line 22 and those remaining memory film portions not under word line 22, becomes "pinned" at about the edge of word line 22. That is, the initial magnetic wall so formed does not move through the tapered end portions toward the ends thereof as it normally does as a consequence of minimizing the magnetic energy. Rather, the wall stays temporarily in place at the edge of word line 22 where it is subject to propagating in the opposite direction when the sense current is next reversed in its direction of flow. The cause of such "pinning" is not known, but is believed to be related, at least in part, to transitions in a significant portion of the sense current flow, first from juncture 14 shorting tungsten layer 20 to flow in films 15, 16 and 17 through the major portion of bit structure 10, and then from there entering the subsequent shorting layer 20 in the following juncture 14. These sense current flow transitions at these locations are thought to have effects which can on occasion lead to such "pinning" of magnetic walls.
The situation is shown in FIG. 3 in one of memory films 15 and 16 with the edges of word line 22 in FIG. 3 being represented by two vertical dashed lines. There, a magnetic wall is shown formed just at the point in the film portion over which the left edge of word line 22 passes. As can be seen there, the magnetizations in the film to the left of the left edge of word line 22 in the central parts of the film point in the downward direction following the magnetization directions established for the prior magnetization state stored in that film. On the right side of the left edge of word line 22, the magnetizations of the central portions of the film point in the upward direction following the direction of the state that is intended to be stored in the film as the result of the word line current flow for the current storage procedure. Thus, a 180.degree. magnetic wall has formed between these two regions in the film under what would be the left edge of word line 22, if it were shown, and also near where one of the current transitions described occurs. Because of being "pinned," the wall has not propagated to the left as normally occurs.
Upon the cessation of the sense and word line currents, this situation shown in FIG. 3 will remain if a "pinning" of the wall shown has occurred. However, a subsequent reversal of the flow of the sense line current may well occur, possibly in the next information retrieval operation attempting to extract information from the memory. Such a sense current reversal will change the sense current transitions situation and the direction of the magnetic field resulting from the sense current, and could thus both permit and urge this previously "pinned" wall to move to the right so that the prior magnetic state is stored again in bit structure 10 rather than the state attempted to be established by the most recent storage procedure. As a result, erroneous information will come to be present in that bit structure.
Thus, there is a desire to store a selected magnetic state in a bit structure 10 that is not subject to becoming erroneous in subsequent memory storage and retrieval operations. That is, there is a desire to store a magnetization state in bit structure 10 that is complete in the sense of having the magnetization established in the central portions of a memory film in a bit structure be in a more or less common direction throughout those portions of the bit structure.